Hot isostatic pressing (HIP) is a technology that finds more and more widespread use. Hot isostatic pressing is for instance used in achieving elimination of porosity in castings, such as for instance turbine blades, in order to substantially increase their service life and strength, in particular the fatigue strength. Another field of application is the manufacture of products, which are required to be fully dense and to have pore-free surfaces, by means of compressing powder.
In hot isostatic pressing, an article to be subjected to treatment by pressing is positioned in a load compartment of an insulated pressure vessel. A cycle, or treatment cycle, comprises the steps of: loading, treatment and unloading of articles, and the overall duration of the cycle is herein referred to as the cycle time. The treatment may, in turn, be divided into several portions, or phases, such as a pressing phase, a heating phase, and a cooling phase.
After loading, the vessel is sealed off and a pressure medium is introduced into the pressure vessel and the load compartment thereof. The pressure and temperature of the pressure medium is then increased, such that the article is subjected to an increased pressure and an increased temperature during a selected period of time. The temperature increase of the pressure medium, and thereby of the articles, is provided by means of a heating element or furnace arranged in a furnace chamber of the pressure vessel. The pressures, temperatures and treatment times are of course dependent on many factors, such as the material properties of the treated article, the field of application, and required quality of the treated article. The pressures and temperatures in hot isostatic pressing may typically range from 200 to 5000 bars, and preferably from 800 to 2000 bars and from 300 to 3000° C., and preferably from 800° C. to 2000° C., respectively.
When the pressing of the articles is finished, the articles often need to be cooled before being removed, or unloaded, from the pressure vessel. In many kinds of metallurgical treatment, the cooling rate will affect the metallurgical properties. For example, thermal stress (or temperature stress) and grain growth should be minimized in order to obtain a high quality material. Thus, it is desired to cool the material homogeneously and, if possible, to control the cooling rate. Many presses known in the art suffer from slow cooling of the articles, efforts have therefore been made to reduce the cooling time of the articles.
In U.S. Pat. No. 5,118,289, there is provided a hot isostatic press adapted to rapidly cool the articles after completed pressing and heating treatment. The press comprises a pressure vessel, having an outer wall, end closures, and a hot zone surrounded by thermal barriers. The outer wall of the pressure vessel is cooled from the outside. The hot zone is arranged to receive articles to be treated. Between the thermal barriers and the pressure vessel with end closures, there are colder spaces, or zones. As in conventional hot isostatic presses, the pressure medium is heated during pressing of the articles, which are placed in the hot zone as mentioned above.
Further, in the press disclosed in U.S. Pat. No. 5,118,289, during cooling of the articles, cooled pressure medium is introduced into the hot zone, whereby thermal energy is transferred from the articles to the pressure medium. Thus, the temperature of the pressure medium will increase during the passage through the hot zone and the temperature of the articles will decrease. When leaving the hot zone, the relatively hot pressure medium will reach the walls of the pressure vessel. In a conventional hot isostatic press, the amount of hot pressure medium reaching the walls of pressure vessel must be carefully controlled in order not to overheat the walls of the pressure vessel, i.e. every interior surface of the press coming in contact with the hot pressure medium. This means that the cooling must be performed at a relatively slow pace, i.e. not faster than the pressure vessel can withstand over time.
The press in the above mentioned U.S. Pat. No. 5,118,289, however, further comprises a heat exchanger, which is located above the hot zone, in order be able to decrease the time for cooling of articles. Thereby, the pressure medium will be cooled by the heat exchanger before it makes contact with the pressure vessel wall. Consequently, the heat exchanger allows for an increased cooling capacity without the risk of overheating the wall of the pressure vessel. Further, as in conventional hot isostatic presses, the pressure medium is cooled when passing through a gap between the pressure vessel wall and the thermal barriers during cooling of articles. When the cooled pressure medium reaches the bottom of the pressure vessel, it re-enters the hot zone (in which the articles to be cooled are located) via a passage through the thermal barrier.
The heat exchanger becomes hot during cooling of the pressure medium and the articles, and, in order to function as a booster during the cooling of articles, the heat exchanger must be cooled before the press may be operated to treat a new set of articles. Thus, a drawback of this type of press is that the time between subsequent cycles is dependent on the cooling time of the heat exchanger. In order to overcome this problem, one approach is to employ two heat exchangers. With two heat exchangers, one heat exchanger may be cooled outside the hot isostatic press, while the other is used in the hot isostatic pressing procedure. However, this results in the drawback of having to exchange the heat exchangers before each pressing operation. Additionally, the use of two heat exchangers, of course, increases costs for the pressing arrangement.